The present invention relates to an electroacoustic transducer such as a speaker, and more particularly to an electroacoustic transducer used in a small electronic instrument such as a portable telephone.
The speaker of the portable telephone has a diaphragm supported on cylindrical case.
In such a speaker, sound discharge holes are provided for discharging sounds in a back chamber on the rear side of the diaphragm. In recent years, there has been provided a speaker in which discharge holes are formed in a peripheral wall of a case.
FIG. 11 is a plan view of a conventional speaker, and FIG. 12 is a sectional view taken along a line XII—XII of FIG. 11.
The speaker comprises an annular frame 1, a yoke 2 made of a magnetic material having a dish shape and secured to the inside wall of the frame 1, a permanent magnet 3 secured to the yoke, and a top plate 4 secured to the upper surface of the permanent magnet 3.
As shown in FIG. 13, the frame 1 has a peripheral annular projection 1a and shoulder 1b. In the projection 1a and the shoulder 1b, four slits 1d and grooves 1c which are communicated with each other are formed at a regular interval as shown in FIG. 11. An annular plate 5 is fixed on the shoulder 1b. A diaphragm 6 having a voice coil 7 is adhered on the annular plate 5 at the periphery thereof, and a protector 8 is mounted on the periphery of the diaphragm and secured to the projection 1a of the frame 1 with an adhesive 10. Thus, a front chamber 11 and a back chamber 12 are formed on the front side and rear side of the diaphragm 6, and a back chamber sound discharge hole 13 is formed in each groove 1c. A plurality of front chamber sound discharge holes 14 are formed in the protector 8.
In the speaker, since there is formed the slit 1d in the projection 1a, the protector 8 is not adhered to the projection 1a at the slit 1d. Consequently, it may happen that the protector 8 is not completely secured to the diaphragm 6, and a gap 16 is formed between the protector 8 and the diaphragm 6 as shown in FIG. 14. As a result, sounds in the front chamber 11 discharge passing through the gap 16. The discharged sound is in opposite phase to the sound discharged from the back chamber sound discharge hole 13. Therefore, both the sounds act to cancel each other, thereby reducing the sound discharged from the discharge hole 14.